Constructing a class in-place

[Johan Engelen]

On Wednesday, 25 July 2018 at 08:11:59 UTC, rikki cattermole 
wrote:
>
> Standard solution[0].
>
> [0] https://dlang.org/phobos/std_conv.html#.emplace.4

Thanks for pointing to D's placement new. This is bad news for my 
devirtualization work; before, I thought D is in a better 
situation than C++, but now it seems we may be worse off.

Before I continue the work, I'll have to look closer at this 
(perhaps write an article about the situation in D, so more ppl 
can help and see what is going on). In short:
C++'s placement new can change the dynamic type of an object, 
which is problematic for devirtualization. However, in C++ the 
pointer passed to placement new may not be used afterwards (it'd 
be UB). This means that the code `A* a = new A(); a->foo(); 
a->foo();` is guaranteed to call the same function `A::foo` 
twice, because if the first call to `foo` would do a placement 
new on `a` (e.g. through `this`), the second call would be UB.
In D, we don't have placement new, great! And now, I learn that 
the _standard library_ _does_ have something that looks like 
placement new, but without extra guarantees of the spec that C++ 
has.
For some more info:
https://stackoverflow.com/a/49569305
https://stackoverflow.com/a/48164192

- Johan